It is well known in the coatings art that significant improvement in adhesion of a film to a substrate can be obtained by incorporating various adhesion promoters into the coating compositions or by employing such adhesion promoters as primers before coating the substrate. In this regard, the use of a wide-range of organosilicon compounds and compositions has greatly benefited the art.
Organosilicon compounds which are useful in these applications generally contain hydrolyzable groups (e.g., halogen, alkoxy) attached to the silicon atom thereof which generate silanol groups upon contact with ambient moisture, and thus readily form chemical and/or physical bonds with mineral and metal surfaces. Also attached to the silicon of the organosilane adhesion promoter is an organic moiety which is reactive with, or at least shows some affinity towards, one of the components of the coating (e.g., a film-forming polymer). In this way, a chemical or physical "molecular bridge" is believed to be formed between the coating and the substrate which results in the observed macroscopic improvement in adhesion. Due to the complex interactions among the substrate, primer and coating composition, however, this broadly stated description has only limited predictive value with respect to any specific system. Several examples of such adhesion-enhancing agents follow.
In U.S. Pat. No. 4,122,127, Mikami et al. disclose adhesion promoters for adhering silicone rubber to substrates. These adhesion promoters consist of an organopolysiloxane resin and an organopolysiloxane copolymer having at least one epoxy group and at least one unsaturated group attached thereto.
In U.S. Pat. No. 4,315,970, McGee describes a method of forming an adherent metal surface on a substrate by treating the substrate with at least one alkoxysilane, or a partial hydrolyzate thereof, having various reactive groups attached thereto. Adhesion of the metal to the substrate is enhanced by the use of the silane.
In U.S. Pat. No. 4,267,297, Hanada et al. teach room temperature curable silicone resins. These materials comprise (A) a hydroxylated organopolysiloxane, (B) an alkoxylated organopolysiloxane, (C) an alkoxysilane and (D) an aminoalkylalkoxysilane. Coatings formed from these resins are said to have excellent heat resistant, water repellency, mold release, weather resistance and storage stability.
Mikami, in U.S. Pat. Nos. 4,283,513 and 4,287,326, teaches resin compositions consisting essentially of a siloxane-modified epoxy resin, an alkoxy functional organopolysiloxane and a curing agent, such as carboxylic acid anhydrides. The compositions are said to have improved retention of electrical properties when exposed to moisture.
Mikami, in Japanese Patent Publication, O.P.I. number 13503/78, Published May 10, 1978, further teaches a silicone resin composition comprising an organopolysiloxane resin, an alkoxysilane having alkyl or alkenyl groups attached thereto, an aminoalkylalkoxysilane and an organic solvent.
In U.S. Pat. No. 4,385,158 to Mikami et al., there is disclosed a room temperature curable, one component epoxy resin modified silicone resin having heat resistance and adhesion to substrates. The silicone resin component of this composition consists of an organopolysiloxane resin, a monovalent organo-substituted alkoxysilane and an aminoalkylalkoxysilane adhesion promoter.
In U.S. Pat. No. 4,343,857 to Uram, there is disclosed a coating for optical members comprising a prehydrolyzed alkyl silicate, a polysilicic acid and a monocarboxylic acid dispersed in an aqueous vinyl interpolymer and an epoxy binder containing a filler of fumed silica or aluminum oxide and a silane. The compositions are said to result in hard coatings for optical systems which are stable with respect to hydrolysis.
Plueddemann, in U.S. Pat. No. Re 32,250, teaches that salts of certain nitrogen or sulfur containing siliconate compositions are useful as stabilizers for aqueous silicates in such applications as treating boiler water, geothermal water and in antifreeze solutions. This patent discusses the precursors of said salts and the preparation thereof from, e.g., the reaction of an aminosilane with either an acrylic acid or ester.
Although the above cited art discloses various silanes in combination with a variety of organofunctional silicone resins, it does not suggest the primer compositions of the present invention which are based on the reaction product of a resinous copolymeric siloxane with certain organofunctional silanes, as described infra.